An image forming apparatus for carrying out an image forming process with a detachably mountable image forming unit mounted thereon, when the mounting of the image forming unit has been detected, whether the mounted image forming unit is a new one is discriminated. Then, unit control information held by an information holding portion for holding unit control information corresponding to the image forming unit is updated on the basis of the results of discrimination at the last time and this time. By such a construction, image formation becomes executable on the mounted image forming unit always in an optimized state.